Contact lenses are classified into hard contact lenses and soft contact lenses. While hard contact lenses were non-hydrophilic and less oxygen-permeable, those having improved oxygen permeability have been developed in recent years. These lenses are susceptible to soil by protein etc. and need to be cleaned, sterilized and preserved daily as with soft contact lenses for an additional effect of maintaining oxygen permeability.
A protein soil attached to the surface of contact lenses can be decomposed and removed by the action of a proteolytic enzyme and various cleaning agents have been proposed and actually used. The proteolytic enzyme is, however, unstable in water and various measures have been employed to overcome this defect. For example, an agent in the form of a solid containing a proteolytic enzyme as a main component, such as tablet, granule, powder or the like is provided and an end-user dissolves same in purified water etc. every time before actual use. This method of dissolving a solid cleaning agent every time of use causes high prices, as well as forces users to undergo troublesome procedure. In addition, said preparation in a solid state tends to suffer from poor solubility. Meanwhile, there have been proposed several methods for stabilizing a proteolytic enzyme in a solution state to provide a treating solution for contact lenses. For example, Japanese Patent Unexamined Publication No. 167726/1989 discloses a preserving solution containing a water soluble polymer having quaternary ammonium group and hydroxyl group, and an enzyme. This preserving solution shows poor cleaning effect and is insufficient as a cleaning solution. Also, Japanese Patent Unexamined Publication Nos. 159822/1988 and 180515/1989 propose a method for stabilizing a proteolytic enzyme by adding the enzyme to a solution containing a water miscible organic liquid. However, the enzyme activity of this solution is extremely low such that its cleaning effect is unpractical. Some of the conventional enzyme-stabilized solutions for contact lenses require diluting for use so as to achieve high enzyme activity. This method has a defect in that the proteolytic enzyme activity is scarce when the concentration of an organic liquid is high and that although the activity can be enhanced upon dilution with water, the stability is degraded.
The present invention has been made in view of the aforementioned situations and aims at stabilizing a proteolytic enzyme in a solution (particularly aqueous solution) and ultimately providing an agent for contact lenses which enables simultaneous cleaning, sterilization and preservation of contact lenses with a single solution.
Another object of the present invention is to stabilize, in producing a lyophilized agent for contact lenses containing a proteolytic enzyme, the proteolytic enzyme during lyophilization of the agent.
A still another object of the present invention is to provide an agent for contact lenses containing a proteolytic enzyme, which permits the proteolytic enzyme to remain stable.
The present inventors took note of the stabilization of proteolytic enzymes capable of removing a protein soil and conducted intensive studies to find that a small amount of a surfactant can achieve stabilization of a proteolytic enzyme in a solution, particularly in an aqueous solution.
As mentioned above, proteolytic enzymes are known to be inactivated in the presence of water. Many surfactants contain water, for which reason a proteolytic enzyme is speculated to be inactivated by the water contained in a surfactant when the two are co-existent, according to the technical level of the field. In light thereof, the aforesaid finding was totally unpredictable.
In addition, the present inventors have found that lyophilization of an agent for contact lenses which contains a proteolytic enzyme results in sufficient removal of water, which in turn affords ensured stabilization of a proteolytic enzyme, and that said lyophilized product is superior in solubility when in use.
In preparing a lyophilized preparation, however, a proteolytic enzyme can be inactivated even when a liquid composition containing a proteolytic enzyme to be subjected to the lyophilization treatment is being prepared, thus making preparation of a lyophilized agent very difficult. To be specific, an aqueous solution containing a proteolytic enzyme which is usually charged in a vial etc. is subjected to a lyophilization treatment to afford a lyophilized preparation. During said procedure, charging of an aqueous solution containing a proteolytic enzyme in a vial etc. and freezing thereof take quite a long time to the point that the proteolytic enzyme is inactivated.
Surprisingly, the present inventors have now found that inactivation of a proteolytic enzyme during the preparation of a lyophilized preparation can be markedly inhibited by the presence of a surfactant.